DESCRIPTION: In this animal research protocol, the investigators propose to ascertain the cerebral blood flow and metabolic effects of the presumed neuroprotective female hormones during and following global cerebral ischemia. Specifically, the proposed study will determine the effect of global cerebral ischemia on cerebral blood flow (CBF), energy metabolism, and pial vessel reactivity in female animals compared to their male counterparts. The investigators also will evaluate whether or not the female hormone, beta- estradiol, plays an important role in recovery mechanisms from ischemia. Un- neutered males also will be studied to determine if there are important gender-specific recovery responses during ischemia/reperfusion and if any therapeutic benefit from reproductive steroid administration is limited to females. The proposed experiments will explore two specific mechanisms of ischemic injury in vivo; specifically, 1) acidosis leading to depressed recovery of energy metabolism, loss of pH regulation, and related iron- catalyzed oxidant injury; and 2) microvascular endothelial dysfunction. To study these variables, the investigators will use magnetic resonance (MR) spectroscopy and intravital microscopy to determine if estradiol acts via specific cellular mechanisms. Four Specific Aims are presented. In Specific Aim #1, the investigators will test the hypothesis that ischemic acidosis is less in females, with consequently more complete recovery of brain energy phosphates compared to males; and that chronic estrogen therapy further improves post-ischemic recovery of energy metabolism and intracellular pH. Specific Aim #2 will examine the effect of pre-ischemic hyperglycemia and its consequent exaggerated tissue acidosis on metabolic recovery, testing the hypothesis that the anti- oxidant activity of estradiol decreases vulnerability to hyperglycemia- mediated reperfusion injury. Intravital microscopy will be employed in Specific Aim #3 to determine if post-ischemic pial vessel reactivity to endothelium-dependent pharmacologic agents is impaired in females and estradiol-treated animals. Finally, Specific Aim #4 will test the hypothesis that chronic estradiol therapy increases brain cGMP, nitric oxide synthase activity and pial vessel responsivity to NO-mediated agents in a dose-dependent manner. The information derived from these experiments should contribute to our understanding of vascular function in females at decreased risk for cerebrovascular disease relative to males and of the role of estrogen as potential neuroprotective therapy for patients of either sex.